Method of preparing subgrades for roads and highways



Aug. 7, 1934. P. HUBBARD 1,969,267

` METHOD 0F PREPARING SUBGRADES FOR ROADS AND HIGHWAYS v Filed-March 17,1930 @fyi . Y Z 4a Y 4@ v w \\v n v 05 104 fl 104] 1066z l0! 104a 105ATTORNEY Patented Aug. 7, 1934 METHOD oF PREPARING `SUBGRADESYFOR ROADSAND HIGHWAYS Prevost Hubbard, White Plains, N. Y.

Application March 17, 1930, Serial N0. 436,525

` 4 Claims.

My invention consists in the novel features hereinafter described.reference being had to the accompanying drawing which illustrates anembodiment of my invention selected by me for 5 purposes ofillustration, and a slight modification thereof, and said invention isfully disclosed in the following description and claims.

The efficiency of any road or highway must depend, to a greater or lessextent, upon the support alforded by the subgrade or natural soil uponwhich the roadway proper rests. The major problem which soil subguardsfor highways presents is the .variation in moisture content. Most drysoils, when properly compacted, will sustain the loads to which highwaysare subjected, so long as they can be maintained free from excessmoisture. `Water linds its Way into subgrade soil mainly by 1. Directprecipitation,

2. Run off and side infiltration 3. Capillary rise from accumulation ofwater which may be drawn upward into the upper portions of the subgradefrom depthsV very considerably belowthe surface.

25 The provision of a waterproof wearing surface and the installation ofadequate drains andculverts will serve to protect the subgrade againstthe infiltration of water from the rst two or of these sources, but willhave no effect whatgrade with excess Water. by capillary action frombelow, which, especially at certain periods of the year, or by frostaction, frequently softens the subgrade material to such an extent as tocause failure of the entire road structure carried upon it under tralicloads. k

In an attempt to prevent the breaking down cf the road structurelocally, due toexcess'moisture drawn into the subgrade soil by capillaryaction, it has been deemed necessary in the more important highways toadopta pavement of such thickness and character as to support thetraffic loads even when the subgrade soil contains the maximum excess ofwater due to capillary action. This often involves enormous expense. ofinitial construction throughout' the j entire length of the road orhighway, and even such roads may fail at certain points where unusual orunexpected quantities of water are drawn into the subgrade soil bycapillary action. Obviously such expensive types of highways arerinapplicable to thousands of miles of roads which must be maintained,and where the provision of adequatefunds for the more expensive ,typesoi roadco'n'struction is impracticable.

ever in preventing the saturation of the subvious that the surfacetreatment, that is to say,

Athe road is subjected, when in moderately dry and compact condition,and to prevent the rise of moisture by capillary action to said layer by'interposing between it and the body of soil be- 55 low, an imperviousseptum whichwill prevent f the access of waterthereto bycapillaryaction,

the said dry compact layer of subgradematerial being further protectedfrom moisture of precipitation by a water-proof wearing surface orpavement, and from side infiltration by proper drains or ditches, soconstructed that the level of water therein shall be below the level ofthe septum. In some instances, where it may be found necessary ordesirable, a plurality of im- 'I5 pervious septums may be employed atdifferent distances below the upper surface of the final subgrade soil,in which case the water level of the drainage ditches will ordinarilybebelow the lowest septum, but for ordinary road con- ,3 0 struction,and especially for firm roads and the like, a single septum below alayer of dry oompact subgrade soil of suitable rdepth will be suiicient.l

In carrying out my invention, it will be ob- 8,5

the actual pavement or roadway which protects the subgrade layer frommoisture of precipita- Vtion and supplies the necessary surface wearingcourse, to prevent indentation by the wheels of 90 Vehicles, may becomparatively thin since the traflic loads are supported directly by thesubgrade material itself, and the cost of road structure is therebyvastly decreased. It is to be understood, however, that my invention isalso 95 applicable to the more expensive types of road as it will notonly relieve them from the present menace of accumulationof water in thesubgrade soil by capillary action, but will permit of the use of apavement of less depth, and

greatly decrease the cost of building the more f expensive types ofroadways, such for example,

as those of concrete, cement, or other like constructiomand thosecomposed of granular stone combined with asphalt, orr other binders,also brick and block.

Referringr to the accompanying drawing, which illustrates onemanner inwhich my invention may be carried into effect, and a slight modiiicationthereof,

Fig. l represents a cross section of the subgrade for a road or highwayprepared in accordance with my invention.

Fig. 2 represents the same subgrade construction, with a relatively thinwearing course or pavement of waterproof character thereon.

Fig. 3 represents a slight modincation of my invention. in which aplurality 'of impervious septums below the drycor'npact subgrade layerare shown. l'

In carrying my invention into effect, a surface of natural soil,indicated at 1, is prepared, repre-f sented by the dotted line,'l -;c,in Figs. 1 and 2, at a predetermined distance below the upper level ofthe subgrade soil material upon which the pavement or roadway proper islto be constructed. rIhis will ordinarily be vaccomplished by excavatingthe roadway to the required depth and removing the natural subgradematerial therefrom, but obviously where filling is necessary, the levelindicatedby the dottedline, m-."c, must be obtained in the usual manner.V1Upon thesurface of the soil, 1, at the level indicated by the dottedline, x--:c, I provide an impervious septum, which is indicated at 2,extending-over the entire surface beneath the roadway, and be tween thedrainage ditch, indicated at 3, with which the .subgrade is tobeprovided. I have found that capillary rise of water in soils of highcapillary action can be checked by a relatively thin layer of bituminousmaterial, or soil mixed l.with or impregnatedwith bituminousmaterial,

and under lordinary circumstances the impervious septum, 2, may beprovided in this manner. For example, I may employ aV cheap grade ofasphaltum road oil, such as is` commonly used for the surface treatmentof earth roads, but I may` use other bituminous compounds with orwithout admixture to soil or other materials.

My invention, however, includes any type of impervious septumwhichwillvprevent the rise vof water therethrough by capillary action,suchas .impervious sheet'material of any kind which will not becomedisintegrated by the action ofthe surrounding earth. Suitablesheetrmaterial provided with a `protecting coating on one or bothYsidesto insure against disintegration or to render it impervious mayalso be employed, but as one of the objects of my invention istoprovidean exceedingly inexpensive and yet durable and satisfactory roadway, Iprefer to employa septum composed mainly or entirely of bituminousmaterial, or such material ,mixed with soil or other granular material.The `dry subgrade material, indicated at '4, is superimposed upon theseptum, 2, to the desired depth. Ifthe level, indicated by the ldottedline, .r-, has been obtained by removing the subgrade material, thissame material may be replaced andthe drainage ditches, 3, areprovidedfof such depth that the normal water level therein will notatvany time rise aboveV the level ofthe septum, 2. The depth of the drysubgrade material, 4, which .will ordinarily' be of exactly the' samecharacter as the soil, 1, below the septum, will depend upon thecharacter of the subgrade soil. Ordinarily the subgrade material will bereplaced above the septum, 2, in exactly the same condition in which itwas before its removal. Obviously, however, in some instances, additionsof sandor clay may be made, if desired, butthis will not ordinarilyber'rnecessary. The. dry subgrade material will be graded and compactedinany ordinary or preferred manner, with or without the laddition ofwater, and the shoulders, indicated at 4a, will preferably be given asuicient inclination toward the respective drainage ditches, tofacilitate drainage of precipitated moisture. This dry compact subgradematerial is then covered with a road surface, or pavement, whichpreferably should be of a water-proof character. In Fig. 2, for example,I have showna thin road surface, indicated at 5, which may be composedof bituminous material, with or without inely` divided stone or othersuitable material, and all in desired depth. The shoulders, 4a, may alsobe coated with water-prooi material, as indicated at V5a, if desired,`although when properly constructed and given the requisite inclinationtoward the drainage ditches, little infiltration is to be feared fromthis source. In the finished roadway, the vwearing course or nishedpavement, whatever its character may be, is supported'upon asubstantially thick subgrade material composed of reasonably dry compactsubgrade soil. beneath which is animpervious 'sepa ,tumv extending thefull width of the roadway, between the side ditches or drains, but at anelevation above that of any standing water that might be expected toaccumulate within them. The dry compact subgrade material, 4, istherefore protected from capillary moisture in the earth, 1, below it bythe impervious septum, 2,

"and, is further protected from accumulation of excess water orprecipitation or lateral infiltration so thatit will remain through theseasons in ics the same condition, and will at all times provide thenecessary support for road traffic.

all directions, including' infiltration from the bottom by capillaryaction, forms no part of the pavement of the roadway, whether comprisingIt `will be understood that the dry subgrade lmaterial soprotected fromexcess moisture in merely an impervious surface treatment ofcomparatively little thickness, or a thicker stratain- Vcluding what maylbe termed foundation material surmounted by an impervioussurface'wear-f ing courseyall of which if used Vwill be located aboveand rest upon the dry subgrade fmaterial so 'protected against moisture.

The depth of the dry compact subgrade layer above the, septum will varyin different soils land 'in Vdifferent localitiesand climates. `Theseptum, 2, may be located either adjacent to or below the normal frostline, if considered necessary, or in many cases, it may be located of aplurality of septumsat different distances from the upper surface of thesubgrade, if this is necessary or desirable, and in Fig. 3 I have`lshown a slight modiiicationy of my invention in which thepartscorresponding to those shown Yin Figs. l and 2 arey given the samereference numeralswith the addition of 100.

In this Yinstance the surface earth isv exca- :v

vated to a greater depth than heretofore described, to wit, toa levelindicated bythe Vdotted line,` y-;1/. The lateral drainage ditches,

103, are carried to a greater depth so that the water level therein willbe below thelevel of the dotted line represented ^by y-y; yThe entirewith an impervious septum, indicatedat 102e, in any of the wayspreviously described. YApportion of the dry subgrade' soil, indicatedat`104b,

surface between the ditches is then provided is then placed on top ofthe septum, 192e, to any desired depth and compacted in any usual orwell known manner. A second septum, indicated at 102, is then providedabove the layer of dry subgrade material, 10411, extending the entiredistance between the drainage ditches, and the remaining portion of thesubgrade material is applied above the upper septum, 102, and compactedand graded to the desired subgrade level. The water-proof Wearingcourse, 105, or roadway, is then applied over the top of the subgradematerial, 104, and may be of any desired depth and of any desiredcharacter, whether merely a water-proof wearing course of bituminousmaterial admixed with earth or broken stone, or any other desired formof pavement may be erected upon the upper layer of subgrade soil, 104.The shoulders, 104e, may also be protected by Water-proof material, asindicated at 105e, extending. downwardly past both of the layers of drycompact subgrade material, 104 and 104].

While this embodiment of my invention involves additional expense, itisobvious that it affords a double protection against infiltration bycapillary action, as any water from thatr source would have to passthrough both the septums, 102 and 102, in order to reach the uppercourse, 104.

While my invention is especially adapted to the preparation of subgradefor roads and highways, it will be understood that it may be applied tothe preparation of subgrade for any sort of pavement, road, airportrunway, taxiway,

landing strip or landing field, or in any other connection in which asupporting surface is to be provided. My invention is especially adaptedfor use in connection with preparation of subgrades for country roadsand the like where a cheap and durable roadway is desired and where theenormous cost of cement and other expensive roadways makes itimpracticable to divert large sums of money to such purposes.

While I have referred to the subgrade material above the septum orseptums as being dry, this does not necessarily mean bone dry, butincludes a condition in which the material may have a moderate degree ofmoisture which will not decrease its load carrying capacity. It willalso be understood that should any excess water penetrate into thesubgrade material above a septum, it will tend to drain out through theshoulders to the lateral drains, and

" the waterproof protection for the shoulders, in-

dicated at 5a and 105e, where employed may be of such character as tofacilitate the passage over them of precipitated water withouthermetically sealing the surfaces. The surface over which the septum orseptums is or are laid may also be crowned, as indicated for example inFig. 3, or inclined or otherwiseconstructed so as to facilitate drainagefrom the superimposed subgrade material should excess water nd accessthereto,v at any time.

What I claim and desire to secure by Letters Patent is:-

1. The method of forming a supporting bed, which consists in placing animpervious substantially horizontal septum upon the subgrade soil at apredetermined depth below the normal subgrade level, then placing a bedof natural subgrade or soil material susceptible to capillary action onsaid septum, having its upper surface below the normal subgrade level,placing an impervious substantially horizontal septum upon said bed ofnatural subgrade or soil material, placing another bed of naturalsubgrade or soil material susceptible to capillary action upon saidsecond septum, providing drainage adjacent to the lateral margins ofsaid beds of a character to prevent a water level adjacent theretohigher than the first mentioned septum, and applying an imperviouswearing surface over said last mentioned bed.

2. A roadway comprising an impervious septum supported in naturalsubgrade soil below the normal subgrade level, a substantially drysupporting bed of naturalsubgrade soil susceptible to capillary actionon said septum, an impervious wearing surface mounted on the supportingbed, drainage ditches at the opposite lateral extremities of said septumhaving a depth suii'icient to prevent a water level above said septum,and a protective coating on the walls of said ditches above and adjacentto the septum for preventing lateral inltration into said bed.

3. The method of forming a roadway which consists in placing animpervious substantially horizontal septum upon subgrade soil at apredetermined depth below the normal subgrade level of the road,superimposing upon said septum a layer of untreated natural subgradesoil of suflicient thickness to support the normal traflic loads when ina dry condition and providing said layer with lateral drainage slopesand providing a protective coating upon the upper surface of said layerof untreated soil, said protective coating extending over said lateraldrainage slopes to their lines of intersection with the plane of theseptum, so as to protect it from water of precipitation and lateralinltration of water.

4. A roadway comprising an impervious septum supported on naturalsubgrade soil below the normal subgrade level, a substantially drysupporting bed of natural subgrade soil susceptible to capillary actionon said septum, an impervious septum supported by said bed below thenormal subgrade level, a substantially dry supporting bed of naturalsubgrade soil susceptible to capillary action on the upper septum, animpervious wearing surface mounted on the upper supporting bed, drainageditches at the opposite lateral extremities of said septums having adepth suicient to prevent a water level above the lower of said septums,and a protective coating on the walls of said ditches above and adjacentto the lower septum for preventing lateral infiltration into said beds.

` PREVOST HUBBARD.

